This invention relates to a composition comprising polycarbonate, wherein the polycarbonate has a low content of a particular branching species. This branching species is commonly known as the xe2x80x9cFriesxe2x80x9d branching species (hereinafter xe2x80x9cFriesxe2x80x9d), and upon treatment yields the molecule shown in formula I. More particularly, this invention relates to low Fries polycarbonate made by the melt synthesis method. This invention further relates to an optical disk comprising said low Fries polycarbonate, and to a method of injection molding which employs said polycarbonate.
Conventional industrial plants synthesize polycarbonate by mixing together an aqueous solution of a dihydroxy compound (e.g., bisphenol-A) with an organic solvent (e.g., dicloromethane) containing a carbonyl halide (e.g., phosgene). Upon mixing the immiscible organic and aqueous phases, the dihydroxy compound reacts with the carbonyl halide at the phase interface. Typically, a phase transfer catalyst, such as a tertiary amine, is added to the aqueous phase to enhance this reaction. This synthesis method is commonly known as the xe2x80x9cinterfacialxe2x80x9d synthesis method for preparing polycarbonate.
The interfacial method for making polycarbonate has several inherent disadvantages. First, it is a disadvantage to operate a process which requires phosgene as a reactant due to obvious safety concerns. Second, it is a disadvantage to operate a process which requires using large amounts of an organic solvent because expensive precautions must be taken to guard against any adverse environmental impact. Third, the interfacial method requires a relatively large amount of equipment and capital investment. Fourth, the polycarbonate produced by the material process is prone to having inconsistent color, higher levels of particulates, and higher chlorine content, which can cause corrosion.
A new method of manufacturing has been developed which avoids several of the problems which the interfacial method. Specifically, some newer commercial polycarbonate plants synthesis polycarbonate by a transesterification reaction where a carbonate diester (e.g., diphenylcarbonate) is condensed with a dihydroxy compound (e.g., bisphenol-A). This reaction is performed without a solvent, and is driven to completion by mixing the reactants under reduced pressure and high temperature with simultaneous distillation of the phenol produced by the reaction. This synthesis technique is commonly referred to as the xe2x80x9cmeltxe2x80x9d technique. The melt technique is superior over the interfacial technique because it does not employ phosgene, it does not require a solvent, and it uses less equipment. Moreover, the polycarbonate produced by the melt process does not contain chlorine contamination from the reactants, has lower particulate levels, and has a more consistent color. Therefore, it is highly desirable to use the melt technique in a commercial manufacturing process. In fact, the assignee has built the world""s first commercial plants which use the melt technique.
The melt technique produces polycarbonate which differs from polycarbonate produced by the interfacial method. Specifically, the conventional interfacial method tends to produce polycarbonate which has close to zero branching. It is desirable to have a low level of branching for some applications, such as those which require very high ductility, but a high level of branching is desireable for other applications which require high melt strength. If branching is desired in polycarbonate produced by the interfacial process, it must be introduced by adding a branching agent during polymerization. Also, the Fries branching species is not typically present in appreciable amounts in interfacial polycarbonate. In contrast, the melt technique tends to produce polycarbonate having a high level of Fries. Accordingly, it would be desirable to produce low Fries polycarbonate by the melt technique for certain applications because higher levels of Fries are associated with low ductility. As noted below, Applicants have solved this problem.
Japanese Published Patent Application Number 9-59371 to Teijin (hereinafter the xe2x80x9cTeijin Publicationxe2x80x9d) discloses a method for manufacturing polycarbonate by the melt process wherein the polycarbonate contains from 0.001 to 0.3 mole percent of Fries plus a second branching species, but contains at least 0.001 mole percent of the second branching species. Therefore, the Teijin Publication specifies melt polycarbonate having a level of Fries below 0.299 mole percent. However, the Teijin Publication does not teach how to make polycarbonate by the melt process which has a very low level of Fries. In fact, the Teijin publication only discloses a polycarbonate made by the melt process having a level of Fries above about 360 ppm (working example 3) and does not mention catalysts which are effective in significantly reducing Fries content. Moreover, the Teijin Publication does not disclose the advantages of using melt polycarbonate having a very low level of Fries in specific applications.
Polycarbonate is widely used in optical disk applications. Examples of optical disks include disks used only for data reproduction, such as the compact audio disk xe2x80x9cCDxe2x80x9d, video disc xe2x80x9cVDxe2x80x9d and CD-ROM formats. Also, newer optical disks can be used for both recording and reproduction. Examples of such applications include write-once optical disks, and erasable writing and reproduction optical disks. Such disks are typically seated into a tray, and slid into the device via an electric servo which moves the disk back into the device. If the disk is improperly seated in the tray, the disk will be subject to lateral stress as it holds open the tray. This occurrence happens often enough such than many manufacturers have modified their equipment to reverse direction, sliding the tray back out if the motor encounters sufficient resistance. In view of this eventuality and others (e.g., bending of mailed optical disks), optical disks must be manufactured with sufficiently high ductility such that they will not snap easily if they are bent. Polycarbonate made by the interfacial process typically has sufficiently high ductility such that it can be used, without modification, in optical disk applications. However, polycarbonate made by the melt process typically has fairly low ductility due to its high level of Fries. Therefore, there is a need for a method to produce optical disks from low Fries polycarbonate made by the melt process. As noted below, Applicants have solved this problem by discovering a way to make low Fries melt polycarbonate having high ductility.
Additionally, a general need clearly exists for a polycarbonate made by the melt process which has a very low level of Fries. A need also exists for a method for making low Fries polycarbonate via the melt process.
The above-described deficiencies of the prior art are substantially overcome in accordance with the present invention. Specifically, the present invention provides a composition comprising polycarbonate wherein the polycarbonate has a very low Fries content (e.g., above 5 ppm and below 360 ppm). The present invention also provides a polycarbonate having a very low Fries content which is made by the melt process. This polycarbonate has high ductility and high impact strength. The invention also provides a method for making these compositions. Another aspect of the invention is an optical disk comprising polycarbonate having a very low Fries content which is made by the melt process. Such optical disks resist breakage due to bending.